Green algae present in waters around Sanibel

As Southwest Florida continues to heal after Hurricane Ian, so do its waterways, but one area is experiencing something that may have you doing a double take.

There are bright, almost neon green, algae in the waters off Sanibel. It’s hard to miss.

“Especially on the causeway, you will notice it when you’re driving across with low tide. But we see it all around Sanibel, even on the beach side. It’ll be growing on one sea walls,” said ​Rick Bartleson, a scientist with the Sanibel Captiva Conservation Foundation’s marine lab. “It’s a macro algae called Ulva.”

Bartelson said it likes to grow on hard surfaces like the boat ramp at Sanibel Captiva Conservation Foundation’s marine lab.

“It’s nontoxic. It’s it frequently grows in patches and dense patches like this,” Bartleson said.

When the conditions are right, it needs spores, a surface to attach to, and Bartleson said there’s not much feeding on this algae right now, like snails, for example.

“So the lack of grazers could be partly due to the red tide,” Bartleson said. “We’ve been having the red tide can kill grazers or slow their growth.”

And the green algae does have plenty of nutrients, so does the red tide in these waters.

“We’ve been having high concentration the last few weeks around the causeway where I’ve been sampling and in Tarpon Bay,” Bartleson said.

Bartelson’s samples showed medium concentrations of red tide out in the mouth of Tarpon Bay and high concentrations around the lighthouse.

The green algae is not shocking this time of year, but the presence of both can be worrisome.

The color of the water can tell us a lot about its quality. “So the red tide algae has a variety of pigments. One of them is red, one of them’s brown, one’s green.”

At the Sanibel Captiva Conservation Foundations marine lab, the red tide is so dense Bartleson can see it on the water. “If you look at those streaks right there. Those are the kinds of they’re under the water. Those are red tide streaks”

“Right here, we have almost 10 million cells per liter yesterday,” said Bartleson.

One million cells per liter is considered a high concentration, but is there another significant red tide outbreak on the horizon? No.

“The wind pushed the red tide up in here to get it started. So it’s, it’s not like growing in here, at this density and incubating and getting more dense, it’s getting pushed in here,” Bartleson said.

Which explains the astronomically high red tide cell counts.

Further out in Tarpon Bay and around the causeway, Bartelson’s test result shows medium concentrations.

If you move your eyes from the brown, red, and green shades of red tide, you see a different shade of green from macro algae called Ulva.

“The decomposing algae can be one of the sources of nutrients for red tide. So that is is a way for the green algae to make the red tide algae worse,” Bartleson said.

The converse is true too. The red tide hurts the grazers, like snails, that control the green algae.

“So in that case, if the red tide helps the green algae and then the green algae decomposed helps the red algae, then that’s a positive feedback loop,” said Bartleson.